Castle Key (Bowser's Castle)
From the Super Mario Wiki, the Mario encyclopedia
The Castle Key is a name given to five different keys in Bowser's Castle in Paper Mario. The first one is held by a Koopatrol on the sideway near the entrance. Mario must defeat it to open the entrance door. The second is found in the lava area to the next. Mario should clear the puzzles there to cool down the lava. After that he can use the key to open the door to make progress. The third is in the water area, which also involves puzzle-solving. The fourth is found in the maze area, where the key is hidden at the end of a fork road. The fifth is in a platform area to the next. Mario should use Kooper or Bombette to lift the platforms that allow him to go to the high place and get the key.
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